1. Field of the Invention
This invention pertains to organosiloxane compositions. More particularly, this invention pertains to organosiloxane compositions containing a novel combination of thermally conductive fillers.
2. Description of the Prior Art
The properties of organosiloxane elastomers and resins make them desirable for a variety of end use applications in the field of electronics. These materials are used to coat and enoapsulate solid state electronic devices and the circuit boards on which these devices are often mounted. To function reliably over a long period of time transistors, integrated circuits and other solid state electronic devices must be protected from contact with moisture, corrosive materials and other impurities present in the environment in which these devices operate. While organosiloxane compositions effectively protect solid state devices from materials that can adversely affect their operation, they typically do not possess the thermal conductivity required to dissipate the large amounts of heat generated during operation of some solid state devices.
The rapidly advancing state of the electronics art in recent years has made it possible to reduce the size of solid state devices in response to a demand for more compact circuitry. As more of these devices are crowded into a given area to decrease the over-all size of the equipment incorporating these devices, the passage of air resulting from convection currents is no longer sufficient to dissipate the amount of heat generated at a rate that will avoid damage to the solid state devices. One method for increasing heat dissipation is to increase the thermal conductivity of the materials used to coat or encapsulate the solid state devices. This increase in thermal conductivity can be achieved by addition of thermally conductive fillers to the coating or encapsulating material.
The prior art teaches addition of a variety of fillers to organosiloxane compositions to increase the normally poor thermal conductivity of cured and grease type organosiloxane compositions. U.S. Pat. No. 4,772,521, which issued on Sept. 20, 1988 teaches mixtures of a particulate silicone rubber and a thermally conductive filler. The mixtures are used as pressurizing media for a thermal molding process. The thermally conductive fillers are particulate metals having a melting point below the temperature at which the mixtures will be used.
The use of various metals as thermally conductive fillers is taught in U.S. Pat. Nos. 4,329,565; 4,279,477; 4,292,223; 4,147,669; and 4,083,622; Japanese Laid Open Application No. 59/43059 and West German Patent No. 3,304.857. Metal oxide fillers are disclosed in U.S. Pat. Nos. 4,685,987; 4,602,678 and 4,604,424.
Japanese Laid Open Application No. 52/125,539 and Research Disclosure No. 259,055, published on November 10, 1985 teach using aluminum nitride as a thermally conductive filler in organosiloxane compositions.
The use of silicon nitride as a thermally conductive filler is taught in U.S. Pat. No. 4,544,696, which issued on Oct. 1, 1985, U.S. Pat. No. 4,588,768, which issued on May 13, 1986, and U.S. Pat. No. 4,584,336, which issued on April 22, 1986.
Boron nitride is disclosed as a thermally conductive filler for organosiloxane materials in U.S. Pat. No. 4,574,879, which issued on Mar. 11, 1986; U.S. Pat. No. 3,499,859, which issued on Mar. 10, 1970 and U.S. Pat. No. 4,763,158, which issued on Aug. 9, 1988.
Using mixtures of various thermally conductive fillers in organosiloxane resins and/or elastomers is taught in the following patents and published patent applications:
Japanese Laid Open Application No. 59/147,033--Combination of fibrous (glass, carbon, metal), scaly (mica, glass flake, metal flake) and powdered (alumina, zirconia, metal, silica) materials;
Japanese Laid Open Application No. 56/144,565 - Mixture of beryllium oxide and silicon carbide;
Russian Patent No. 771,138--Mixtures of boron nitride, zinc oxide and powdered aluminum metal;
Japanese Laid Open Application No. 56/837--mixtures of (1) boron nitride and (2) alumina, silica, magnesium oxide, zinc metal and/or mica;
An advantage of aluminum nitride is its ability to be produced with an average particle size of less than one micron and remain as discrete particles without agglomerating when combined with the ingredients of an organosiloxane composition. The disadvantage of aluminum nitride is the inability to incorporate more than about 72 percent by weight of this material into a liquid organosiloxane without adversely affecting the ability of the resultant composition to be applied as a smooth, coherent coating or a non-crumbling potting material.
One objective of this invention is to utilize the small particle size and good thermal conductivity of aluminum nitride in organosiloxane compositions. Another objective is to increase the thermal conductivity of organosiloxane compositions relative to what is achievable using prior art fillers while retaining the utility of these compositions as coatings, encapsulants and potting compounds for solid state electronic devices.